A problem which can affect the accuracy of such counters is that of "dead time", i.e. the presence of periods in which effective counting is disabled for one reason or another, giving rise to errors or ambiguities in the counter output. One well-known form of dead time, often presented by detectors of radiation events such as Geiger-Mueller tubes, is that detection of an event disables a detector for a finite period so that a further immediately subsequent event may not be detected. The present invention is not concerned with the avoidance of dead time in such detectors, but because such devices generate "counts" which are counted, and are thus themselves frequently known as counters, it should be made clear that the counters with which the present invention is concerned are devices produced by such a detector device. Thus, in the specification that follows, the word "counter" is utilized to refer to a device that counts events, and not to a device which generates counts to mark such events.
Counters that count events occurring within a period also commonly provide a source of dead time, particularly multiple stage digital counters utilized in electronic counting instruments where allowance must be made for the time required for the count to "ripple through" a multiple stage counter. The nature of this problem and a discussion of various solutions which have been proposed are to be found in U.S. Pat. No. 4,519,091 issued May 21, 1985 to Chu et al. The solution proposed by Chu et al utilizes an uninterruptible counter having a synchronous front end, followed by a ripple counter and associated with a synchronizer to compensate for ripple through effects in the ripple counter. U.S. Pat. No. 4,395,762 issued July 26, 1983 to Wondergem et al discloses a further uninterruptible counter arrangement, in this case intended for counting events at a fixed frequency within periods set by events whose periodicity is to be measured. Pulses to be counted are applied to a ripple counter serially through a multiple stage shift register, parallel outputs from which are utilized to time the operations of the counter so that a fixed, known number of pulses will be lost each time the counter is stopped, read and reset.
In both arrangements, the use of synchronizer circuits entails restraints in that the event frequency range over which such arrangements can operate is necessarily limited both as to range and upper frequency limit. Both systems thus require a prescaler on the event input, which in the case of the Chu et al arrangement must also be synchronous, and in the Wondergem et al arrangement must be programmable, thus limiting the upper input frequency range. These systems therefore may be characterized as "synchronized" counters, rather than fully synchronous, but disadvantages of synchronous counters are only partially overcome, and they are not well suited for applications in which counting of the events over a very wide range of input repetition rates is required, with an upper end at a level at which the use of complex counter arrangements at the counter input is impracticable or uneconomic.
A further type of dead time which frequently occurs in counter arrangements, including those discussed above, is dead time whilst the counter output is updated; there will usually be a finite time during updating when the output is either unreliable or undefined.